The present invention relates to medical imaging systems in general, and to ultrasound imaging systems in particular.
Ultrasound imaging is a commonly used technique to non-invasively obtain images of a patient""s internal body matter. One specification of an ultrasound system that directly relates to the quality of images that can be produced is the point spread function of the transmit beam used to apply ultrasound signals to the patient""s body. To achieve the highest contrast resolution, the ultrasound beam that is transmitted into the patient should have as low sidelobes as possible.
One method of improving the quality of ultrasound images is to transmit signals from an ultrasound transducer with an apodization function that reduces the acoustic power of the signals transmitted from the sides of the transducer while allowing an increased acoustic power to be transmitted from those transducer elements near the center of the transducer. This has the effect of reducing the sidelobes in the transmit beam. In the past, such an apodization scheme was implemented using a separate voltage regulator in line with each individual piezoelectric crystal in the transducer. This approach is costly.
Given this problem, there is a need for an ultrasound transmitting mechanism that can improve the point spread function of a transmit beam without requiring individual voltage regulators in line with each transducer element.
The present invention is a system for simulating a transmit apodization without the use of voltage regulators in line with each transducer element. In one embodiment of the invention, individual transducer elements are selectively turned on or off such that each transducer element either transmits a pulse or does not. The selection of which transducer elements transmit a pulse is determined in accordance with an apodization probability function such as a Hamming function. With the probability function, fewer transducer elements near the edges of the transducer transmit a pulse while a greater number of transducer elements near the center of the transducer are selected to transmit a pulse. As a result, the combined transmit pulses from all the energized transducer elements simulate an apodized transmit beam.
In another embodiment of the invention, each transducer element transmits a variable portion of a transmit pulse. Transducer elements at the outer edges of the transducer transmit a lesser percentage of a pulse than those transducer elements located at the center of the transducer. A waveform generator associated with each transmitter element is supplied with parameters including the frequency of a pulse, the number of cycles to be transmitted, and a delay calculated for that element. From these parameters, the waveform generator synthesizes a variable portion of a transmit pulse to be transmitted.
The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:
FIG. 1 illustrates the operation of a conventional ultrasound transmission system that transmits a number of pulses with no apodization;
FIG. 2 illustrates a conventional ultrasound transmission system having voltage regulators in line with each transducer element to implement an apodization in a transmit beam;
FIGS. 3A-3C illustrate an ultrasound transmission system for selectively transmitting a pulse from fewer than all the number of transducer elements in accordance with the first aspect of the present invention; and
FIGS. 4A-4B illustrate an ultrasound transmission system in accordance with another aspect of the invention wherein each transducer element transmits a variable portion of a pulse.